In the field of detectors used for infrared imaging or thermography (pyrometry), the use of devices configured in the form of an array and capable of operating at ambient temperature, i.e. not requiring cooling to extremely low temperatures, is known—in contrast to detecting devices referred to as “quantum detectors” which can only operate at extremely low temperature. Generically, uncooled detectors are referred to as “thermal detectors”.
These detectors traditionally use the variation in a physical value of an appropriate material or an assembly of appropriate materials as a function of temperature at around 300 K. In the particular case of the most widely-used bolometric detectors, this physical value is electrical resistivity, but other values such as dielectric constant, polarization, thermal expansion, refractive index, etc. can be used.
Such an uncooled detector generally includes:                means of absorbing the thermal radiation and converting it into heat;        means of thermally isolating the detector so that its temperature can rise due to the effect of the thermal radiation;        thermometric means which, in the context of a bolometric detector, use a resistance element, the resistance of which varies with temperature;        and means of reading electrical signals provided by the thermometric means.        
Detectors designed for thermal or infrared imaging are conventionally produced as a one- or two-dimensional array of elementary detectors, said detectors being suspended above a substrate, which is generally made of silicon, by means of support legs.
The substrate usually incorporates means of sequentially addressing the elementary detectors, means of electrically exciting the elementary detectors and means of pre-processing the electrical signals generated by these elementary detectors. This substrate and the integrated means are commonly referred to as the “readout circuit”.
In order to obtain a scene using this detector, the scene is projected through suitable optics onto the array of elementary detectors and clocked electrical stimuli are applied via the readout circuit to each of the elementary detectors or to each row of such detectors in order to obtain an electrical signal that constitutes an image of the temperature reached by each of said elementary detectors. This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
An elementary detector is formed by a thin membrane (of the order of 0.1 to 0.5 μm) fixedly held suspended parallel to the substrate with the aid of thermally isolating support structures usually referred to as “legs”. At least some of these structures also act as an electrical link between the contacts made on the surface of the readout circuit and the electrically active parts of the membrane. In addition to the sensitive material, the membrane consists of materials used in order to maximize absorption of the thermal radiation to be detected, for example using a conductive layer having an appropriate sheet resistance, usually in conjunction with a reflector located on the surface of the substrate. This reflector is designed to increase absorption in the vicinity of a given wavelength, usually between 8 and 14 μm, due to the quarter-wave effect. The gap between the membrane and the reflector is consequently adjusted to around 2 to 2.5 μm. These types of construction are very familiar to those skilled in the art.
The essential performance of such a detector is expressed by its thermal resolution or NEDT (stands for Noise Equivalent Differential Temperature). This quantity is primarily determined by the thermal resistance Rth seen between the membrane and the substrate which is kept at a temperature that is essentially constant.
This quantity Rth is essentially defined by the constituent materials and the geometry of the support legs. One of the ends of these legs is integral with the body of the membrane and the other end of these legs is integral with the substrate via an intermediate anchoring structure. Quantity Rth is first-order proportional to length and inversely proportional to the width and thickness of the legs (assuming they are made of a single material for the sake of simplicity). It is therefore preferable to use materials which have high thermal resistivity and are very rigid as constituent materials of the legs. Silicon nitride is highly suitable from this point of view and is therefore very widely used together with a very thin (several nanometers) electrically conductive layer which is necessarily integral with at least two legs per elementary detector in order to ensure electrical continuity between at least two connection points formed on the surface of the readout circuit and the electrically active structures of the membrane of the elementary detector.
The main problem encountered in obtaining optimal performance is that of defining maximum thermal resistance while ensuring satisfactory geometrical stability of the suspended sensitive membrane. In fact, reducing the thickness and width of the legs and any increase in their length quickly reaches a limit beyond which rigidity becomes insufficient. In other words, elastic deformation becomes excessive and makes it impossible to accurately secure the membrane over the substrate, given the fact that the thickness of the quarter-wave sheet must be essentially uniform in order to obtain uniform, constant spectral responsivity from one detector to another.
The usual solution adopted to solve this problem is to concentrate on the length parameter by increasing the length of the legs between their attachment point and the point at which they merge into the membrane along one or two or even more adjacent edges of the membrane. This twisted confirmation necessarily requires the use of materials which are relatively thick in the field in question (several hundred nanometers) with a width of at least the same order in order to effectively support a typical membrane having an edge size of 25 μm. As a result, this concept is intrinsically limited in terms of the Rth value which can be achieved in practice because of the resulting loss of rigidity.
What is more, this arrangement limits the fill factor of the structure which expresses the efficiency with which it collects the radiative energy that is to be detected. In fact, legs that are thus lengthened inactivate, at least partially because this is not their primary objective, part of the footprint of the elementary detector and this inactivation is proportionally greater relative to the total surface area if the legs are extended.
One solution to this new problem involves dedicating a first constructional level to lengthening the legs, typically in the form of coils which snake to and fro parallel to one edge as disclosed, for instance, in documents U.S. Pat. Nos. 6,034,374, 6,094,127 or 6,144,030. The absorbent, sensitive membrane is formed at a second superposed constructional level and is connected to the end of the subjacent legs opposite the points where they are anchored to the substrate. This results in high thermal resistances but unlimited reduction of the thickness and width of the legs nevertheless remains impossible because of the mechanical reasons mentioned above and which are exacerbated in this case. Also, because the presence of legs between the substrate and the membrane interferes with the quarter-wave resonance effect, it becomes necessary to complicate the structure considerably by producing:                either legs with high reflectance in order to produce the reflector effect on the actual legs. This effect is bound to be imperfect and results in an increase in the thermal conductance of the legs which is contrary to the sought-after objective;        or by interposing a reflector supported by its own anchoring points and provided with perforations which, once again, are contrary to the sought-after objective at a third intermediate constructional level between the legs and the membrane. A construction of this type is described, for example, in document US 2002/0179837A1.        
These extremely complex structures result in devices that are expensive to fabricate, firstly due to the very large number of technological processes that need to be performed and the unavoidably low yields associated with them and secondly due to the technological dispersion caused by the multiplicity of processes that they involve.
One solution which imposes far fewer restrictions in this respect involves only constructing linear legs with one of their ends being secured by anchoring structures designed to physically attach to the substrate, the other end being integral with the body of the membrane and assembling these legs in collinear pairs. FIG. 1 shows a typical example of this type of construction in accordance with the prior art. It is understood that at least two legs must be electrically conductive and in continuity with the connections formed on the surface of the readout circuit so as to address the resistance formed in the membrane. A structure of this type with only two legs is described, for example, in U.S. Pat. No. 5,021,663. Mechanical retention of membranes is then valid even for much thinner leg thickness (of the order of 15 to 50 nm for usual materials and pixels having an edge dimension of around 25 μm) without making it necessary to complicate the structure or to reduce the fill factor substantially. The fill factor remains high because the legs (or pairs of legs arranged along a common axis) do not exceed the length of an edge or a diagonal of the membrane.
It is then possible to obtain very high Rth values with a good fill factor without resorting to complicated constructions. However, the limitations associated with this prior art reappear with small juxtaposition pitches which are encountered in the case of elementary detectors used to form arrays, typically below 25 μm. Firstly, because the anchoring points on the substrate are necessarily relatively solid and essentially arranged along the axis of the legs for mechanical reasons, their overall size limits the linear length of the legs. Also, the anchoring points are usually formed with a lateral extension (in both dimensions parallel to the plane of the substrate) which becomes non-negligible with these very small pitches relative to the surface area available on the footprint of an elementary pixel. This results in a reduced membrane fill factor.
An estimate of the construction constraints encountered is given below in relation to FIGS. 1 and 2. In the most usual case of detector arrays having a pitch of 25 μm, assuming favorable, practical cases where the anchoring structures are each common to two adjacent detectors, as shown in FIG. 1, and assuming that an anchoring structure 4 occupies an area of roughly 5×5 μm, pairs of legs 3 can be extended by a total of approximately 15 to 17 μm, assuming a gap of 3 to 5 μm has to be left in order to attach the membrane to each pair of legs. This results in the formation of satisfactory thermal resistance compared with the other construction possibilities mentioned above, even though each membrane according to FIG. 1 is supported by four legs rather than two legs as in another embodiment of the prior art such as that shown, for example, in Document U.S. Pat. No. 5,021,663.
Transferring this exercise to an array repetition pitch of 17 μm, the total extension of the pairs of legs must not exceed 9 to 11 μm, although it is relatively difficult to reduce the widths and thicknesses because they are imposed by other technological constraints. Also, the useful surface area for extending the membrane which collects radiative energy relative to the area of the elementary detector is reduced by around 10 to 15% and this is penalizing because the aim is always to achieve maximum performance. The budget, in terms of sensitivity, is reduced by 40 to 50%. Given the fact that the incident radiative energy on the surface area of an elementary detector is already reduced by a factor of more than 2 when two detectors having respective pitches of 25 and 17 μm are compared, these findings show that attempting to achieve sensitivity with very small array pitches is highly problematic.
The crucial need to achieve performance gains for small array pitches, preferably without complicating their structure, is therefore readily apparent.
This geometrical limitation of the anchoring structures is associated firstly with the need to obtain, reliably and statistically dependably, electrical connectivity between the potentials monitored by the readout circuit and the electrical functions of the membrane via the support legs and secondly with the need to ensure mechanical rigidity of the assembly relative to the substrate. Producing the anchoring structures requires the use of several materials, each of which must be defined by a particular lithographic and etching process which meets printed circuit artwork rules which cannot be relaxed arbitrarily. A final overall size of around 5×5 μm is comfortable and 4×4 μm obtained using more stringent techniques and inspections represents a good realization compromise using ordinary tried-and-tested professional techniques.
The object of the invention is therefore to propose thermal detectors of simple construction which offer performance better than that of detectors according to the prior state of the art, regardless of the array pitch and, in particular, with small array repetition pitches. The invention solves the problem of overcoming the limits which prevent the realization of high thermal resistances, not only at the level of structures for anchoring to the substrate in a first embodiment, but also and possibly cumulatively, at the level of integral attachment of the legs to the sensitive membrane in a second embodiment. The concept can be extended in accordance with a third embodiment which gives an even greater improvement in sensitivity.